This proposal addresses an important problem in vascular cell biology: the influence of hemodynamic shear stress on the regulation of vascular tone. We propose that angiotensin I-converting enzyme (ACE) plays a pivotal role in the modulation of vascular wall tonus, and that the expression and activity of this enzyme might be regulated by hemodynamic shear stress. There is reasonable precedent for this idea in the observations by others that hemodynamic forces alter other endothelial functions, such as production of prostacyclin and plasminogen activator. ACE is a zinc-containing molecule found as a soluble enzyme in the plasma, and on the luminal surface of both the vascular endothelium and endothelial cells in culture. ACE converts angiotensin I to the vasoactive angiotensin II by cleavage of a dipeptide from angiotensin I. Angiotensin II is a potent vasopressor and a powerful secretogogue of aldosterone, a hormone important in the maintenance of the sodium balance in kidneys. ACE also hydrolyzes and inactivates the vasodilator bradykinin. Because of its action on both angiotensin II and bradykinin, ACE has a very important pivotal role in the regulation of blood pressure and transvascular fluid exchange. The study will use endothelial cells in culture to investigate the influence of hemodynamic forces on the synthesis, degradation and secretion of ACE under quiescent and shear stress stimulated conditions. We will examine the effect of flow-induced mechanical shear stress by subjecting cells to varying levels of shear stress in a parallel plate flow chamber. Cells subjected to these conditions will then be assayed for ACE activity using a radiolabeled synthetic substrate and monoclonal antibodies developed by our laboratory.